1. Field of the Invention
The present invention relates to a control apparatus for a pulse width modulation inverter, and more particularly to a control apparatus having an improved changeover function between a three pulse mode and a single pulse mode of an inverter output voltage.
2. Description of the Related Art
Although the details will be described later, in a pulse width modulation inverter (called a PWM inverter, hereinafter), the number of voltage pulses in the half cycle of the operation period of the PWM inverter is varied during the control of the output AC voltage thereof. When it becomes necessary to increase the inverter output voltage, at the last course of control, the number of the output voltage pulses in the half cycle of the operation period of the inverter must be changed over from three pulses to one pulse in order to obtain the highest output voltage possible, that is, the operation of the inverter has to be switched from a three pulse mode to a single pulse mode.
One of the control technique for the PWM inverter of this kind is disclosed in the Japanese Patent Laid-open No. 57-132772, for example. According thereto, a control system is intended to solve the following problems in an adjustable voltage/adjustable frequency PWM inverter, i.e., the undesirable change in the inverter output voltage upon changeover between a three pulse mode and a single pulse mode, and the phase deviation of the fundamental component of the inverter output voltage at that time. This control system, however, is not yet devoid of problems that the control is complicated and the phase of the fundamental component of the inverter output voltage still changes during the course of the phase control which is conducted with three pulses for the changeover from the three pulse mode to the single pulse mode and vice versa.
In Japanese Patent Laid-open No. 57-85583, there is disclosed a control method of obtaining gate signals of a PWM inverter by comparison of a triangular wave as a carrier wave and two sinusoidal waves as modulating waves which have the phase difference of 120.degree. from each other. Also in this method, however, the phase deviation occurs in the fundamental component of the inverter output voltage at the time of changeover between a three pulse mode and a single pulse mode. Further, in this method, it is very difficult due to the inductance of a motor supplied by the inverter that the inverter output voltage in the single pulse mode is determined in advance to be almost equal to the maximal inverter output voltage in the three pulse mode, and therefore the undesirable change in the inverter output voltage can not be suppressed.
On the other hand, the Japanese Patent Publication No. 60-24670 discloses a method of generating a multipulse current in every half cycle of the fundamental component of the output current in a current source inverter. However, this prior art only concerns the reduction of higher harmonic components included in the output current and does not disclose that the number of current pulses is changed over for controlling the output current of the inverter. Therefore, although the waveform of gate signals similar to those according to the present invention is shown, there is nowhere in the prior art the description of the change in the output voltage or current and the phase deviation which occur at the time of changeover between a three pulse mode and a single pulse mode of the output voltage or current of an PWM inverter.